Abstract The promise of harnessing cell-mediated (TH1) immunity to treat cancer or establish prophylactic immunity against chronic viral pathogens like HIV continues to be hampered by an incomplete understanding of many different critical factors and processes, including the manner by which the innate and adaptive immune systems interface and communicate. CTLA-4 is one of the best characterized of the immune checkpoint proteins, molecules that serve to balance, regulate, and fine-tune immune activation with homeostatic inhibition. The critical importance of CTLA-4 to the regulation of T-cell activation and proliferation was demonstrated nearly two decades ago by the dramatic phenotype of the null mutant, animals which die in the early postnatal period of uncontrolled lymphoproliferation and autoimmune inflammation. CTLA-4 is expressed by all major lymphoid lineage effectors including T, B, and NK cells; however, its functionality has been best characterized in T-cells where it exhibits both cell-extrinsic and cell-intrinsic regulatory functions. There is almost nothing known about CTLA-4 expression or function in non-lymphoid cell types, and sporadic reports of CTLA-4 expression in non- lymphoid lineages have been inconclusive. Recently our group reported expression and secretion of microvesicle-bound CTLA-4 from myeloid lineage innate dendritic cells (DC). Microvesicular CTLA-4 demonstrat ed the ability to bind B7, leading to vesicle internalization and subsequent downregulation of B7 expression. Knockdown of DC-expressed CTLA-4 resulted in a dramatic upregulation of CD8+ cell proliferation as well as enhanced antitumor and antiviral immunity in vivo. The discovery of non-lymphoid CTLA-4 expression in an innate cell type and concomitant characterization of novel function signifies a significant paradigm shift in the understanding of CTLA-4's role in immune governance as well as the manner by which the innate and adaptive arms of the immune system communicate. Successful completion of the work proposed herein will characterize the basic mechanisms by which DC expressed CTLA-4 is regulated as well as the manner by which this critical immunoregulatory molecule contributes to crosstalk between innate and adaptive immunity. These data will enable and accelerate development of novel therapeutic approaches and assist in enhanced exploitation of existing therapeutics (i.e. ipilimumab) that target CTLA-4 checkpoint pathways.